CANADIAN PERSPECTIVE ON EYE DOSE & INTERNATIONAL RECOMMENDATIONS PRESENTED TO: Presented To:

Transcription

1 CANADIAN PERSPECTIVE ON EYE DOSE & INTERNATIONAL RECOMMENDATIONS PRESENTED TO: Presented To: ISOE ALARA SYMPOSIUM HYATT REGENCY PIER 66 FORT LAUDERDALE 11 JANUARY 2016 Douglas Chambers Ph.D. Arcadis Canada John Chase CHP Sr. Technical Expert External Dosimetry Ontario Power Generation

2 Outline of Presentation Background Considerations in developing dose limits for the lens of the eye Observations on current operational dosimetry Regulatory Considerations Conclusions

3 System of Radiological Protection Most recently updated in ICRP Publication 103 (2007) Based on science, value judgments, and experience Forms the basis of radiation safety standards, legislation, guidance, programmes, and practice worldwide

4 Protection of Human Health Manage and control exposures so that: Deterministic effects (harmful tissue reactions) are prevented The risks of stochastic effects (cancer or heritable effects) are reduced to the extent reasonably achievable ICRP 103 recent studies have suggested that the lens of the eye may be more radiosensitive than previously considered

5 Tissue Reactions ICRP Publication 118 ICRP Statement on Tissue Reactions Early and Late Effects of Radiation in Normal Tissues and Organs Threshold Doses for Tissue Reactions in a Radiation Protection Context

6 ICRP s Statement on Tissue Reactions The Commission continues to recommend that optimisation of protection be applied in all exposure situations and for all categories of exposure. With the recent evidence, the Commission further emphasises that protection should be optimised not only for whole body exposures, but also for exposures to specific tissues, particularly the lens of the eye, and to the heart and the cerebrovascular system.

7 ICRP 118 Conclusions for Cataract Induction Threshold for acute exposure: ~0.5 Gy with 95% CI including zero Threshold for protracted exposure: ~0.5 Gy Evidence pertains mainly to opacities rather than cataracts because follow-up times were generally shorter Newer study* from RERF: At 1 Gy, 20-30% excess of cataract surgery Threshold of 0 to 0.8 Gy, if one exists * Neriishi K, Nakashima E, Minamoto A, Fujiwara S, Akahoshi M, Mishima HK, Kitaoka T, Shore R: Postoperative cataract cases among atomic bomb survivors: Radiation dose response and threshold. Radiation Research 2007; 168:404-8

8 ICRP s Statement on Cataracts cont d For occupational exposure in planned exposure situations the Commission now recommends an equivalent dose limit for the lens of the eye of 20 msv in a year, averaged over defined periods of 5 years, with no single year exceeding 50 msv. Proposed new limit of 20 msv Numerically identical to Effective Dose Limit Eye Dose limit more restrictive than Effective Dose limit: Gamma dose to head higher than to trunk, or Beta dose (> 800 kev) is significant Given the substantially lower (a factor of 7.5 lower) threshold, a higher limit would not be adequately protective Alignment with the effective dose limit facilitates implementation

9 The Eye PSC cataracts historic sentinel of radiation exposure but also seen in cortical opacities Picture credit: -catract1.jpg

10 ICRP s New Recommendations Observations from recent studies More people at low exposures and longer follow-up (epi studies) deterministic effects now referred to as tissue effects since some of the deterministic effects may take years to develop Increased risk of cataracts at cumulative doses above 500 msv No effect of dose rate or fractionation Uncertain threshold ( 0.5 Gy) Assumes LNT

11 Cataracts Most frequent cause of blindness worldwide Classified by location : nuclear, cortical and posterior subcapsular ( psc) Multifactorial Causes Oxidative stresses age related effect -most cataracts advance after the age of 45, but most cataracts are treatable by surgery Genetic factors Other factors include: Sunlight, alcohol intake, smoking, diabetes, use of corticosteroids Ionizing radiation

12 Opacity with Age (Visual impairment in 50% SSK, 2009

13 Prevalence of Cataracts in Canadian Males Prevalence (%) Canada Newfoundland PEI Nova Scotia New Brunswick Quebec Ontario Manitoba Saskatchewan Alberta BC Age (years)

14 Prevalence of Cataracts as a Function of Age in Males in US 70% 60% 50% Prevalence (%) 40% 30% 20% White Black Hispanic Other 10% 0% Age

15 Human Studies Atom Bomb survivors Clinical exposure Chernobyl cleanup workers Occupational (e.g., radiologic technologists) Commercial airline pilots Spaceflight A few examples follow:

16 Radiation Technologists Chodick et al., 2008

17 Bomb Survivors Neriishi et al., 2007

18 Bomb Survivors (cont d) Neriishi et al., 2012

19 Threshold Estimates Over the Years Source Threshold estimate End Point 16 Anecdotal, pre Gy Cataracts 14 Cogan and Dreisler (1953) 6 Gy Cataracts 12 Merriam and Fochet (1957) 2-5 Gy Cataracts 10 Otake and Schull (1982) 1.2 Gy Lens Opacities Dose (Gy) 8 6 UK National Radiological Protection Board (NRPB) (1996) German Radiation Protection Board (SSK) (2007) Gy Cataracts 2 Gy Cataracts Anecdotal, pre-1950 Cogan et al. (1953) Merriam et al. (1957) Otake et al. (1982) NRRP (1996) SSK (2007)

20 Dose Response 5 OR/RR/HR Vs. Dose 4 OR/RR/HR 3 2 Hazard Ratio Relative Risk Odds Ratio Dose (Gy)

21 Replot of Neriishi 2012 Effect of Hypothetical Dose Uncertainty (Upper and Lower Range adapted from Neriishi, 2012) Increment Rate (%) Dose (Gy) Neriishi et al., 2012, Radiology, October

22 Industry Response COG Workshop Radiobiology, Regulation, and Dosimetry of the Lens of the Eye (Oct 2013) COG SOR Radiobiology and Regulations for Eye Doses in CANDU Facilities Comments on CNSC Discussion Paper re Changes to RPRs COG SOR Routine External Dosimetry Eye Dosimetry in CANDU Facilities Utility specific activities Co-ordination and collaboration with EPRI, WNA, etc.

23 Industry Response cont d COG Report COG , Radiobiology and Regulations for Eye Dose in CANDU Facilities, Doug Chambers, November 2014 EPRI Report EPRI 2014 Technical Report, Epidemiological and Mechanistic Effects of Radiation on the Lens of the Eye Industry comments to IAEA,CNSC, Ongoing COG and industry research, e.g., Technical note on dosimetry e.g., H P (3) dose coefficients not well developed (especially for betas) OPG/BP WB TLD Badge for eye dose Beta spectra in CANDU plants

24 Canadian Regulation (1) The CNSC developed a discussion document which considered ICRPs publication 118 on Tissue Reactions in making their decision to propose a reduction in the dose to the lens of the eye the CNSC s Regulatory discussion document and links to associated materials can be viewed which can be obtained at This web site also provides links to associated documents and comments provided to the CNSC

25 New ICRP Recommended Occupational Dose Limits for Lens of the Eye Previous New 1,2 150 msv per year 20 msv per year, averaged over defined periods of 5 years No single year exceeding 50 msv 1) Assumed threshold at the maximum amount of radiation that a tissue can withstand without developing clinical signs of injury in more than few percent of individuals and the criterion is usually taken from <1% (for cases of induced paralysis) up to a few percent (for less severe and treatable injuries) 2) Accepted by the IAEA and incorporated into IAEA s Radiation Protection and Safety of Radiation Sources: International Basic Safety Standards Interim Edition (2011)

26 Canadian Regulation (2) A few potential Issues include amongst others: Need to develop or identify appropriate dosimetry service Potential beta sensitivity for Hp(3) dosimeters Record keeping at licencees and with Canada s National Dose Registry Communicating implications (and context) to nuclear workers

27 Conclusions (1) Determining dosimetry requirements (when, how accurate) for lens of eye is a challenge Using existing Skin and WB dosimeter results to estimate eye dose is problematic Examination of past skin dose results can place some limits on past eye dose Proper evaluation requires knowledge of beta fields and sophisticated evaluation of dosimeter element response

28 Conclusions (2) Current limits for the lens of the eye are too high but The risk of cataracts seems over-weighted compared to other risks (cancer) Substantial effort will be required to meet ICRP recommended eye dose limit, examples: Dosimetry Record keeping communications